In my opinion, a major reason for this would be peer pressure exerted by the other members of the community.

In many communities, the police are not very popular. Sadly, this is especially true for many of the communities (often poor and non-white) in which community policing might be most needed. In these communities, there is a great deal of crime and the police often have trouble telling the criminals from law-abiding people.

Even so, the law-abiding people are often reluctant to help police. This is partly for fear of retribution from criminals. But it is also partly because the police are often seen as a sort of occupying army that tends to abuse the members of the community.

Civil and criminal liability. The police are given training on what actions constitute crimes, what observations or items are evidence of those crimes, how to collect and preserve that evidence, and how to lawfully confront, arrest, and restrain the suspected criminal. Civilians lack the benefit of such training and their actions will be viewed with a great deal more suspicion than the actions of the police by the authorities (by "authorities", I mean police and prosecuting attorneys). Let us consider an example:

Average Joe sees a large man removing a television from his neighbor's garage late at night. The neighbor is nowhere to be seen and the large man appears to be trying to move very stealthily. Joe is not himself a very large man and he has forgotten his cell phone at home. He does, however, have the baseball bat he foolishly decided to bring with him for his neighborhood watch duty. Joe believes his neighbor's house is being burglarized by a large and (quite possibly) dangerous man, so he sneaks up behind the man and hits him on the head with his bat. The TV falls and breaks. To Joe's horror, the neighbor runs out of the garage screaming about his cousin who was helping him move. The large man was moving quietly because it was late. So Joe has now committed assault (quite possibly assault with a deadly weapon or assault with intent to do great bodily harm, depending on the statutes in his jurisdiction), he has caused property damage, and his best defense is that he was trying to protect his neighbor's property. What Joe doesn't know is that his legal defense is going to fail because attacking someone with a bat is not going to be considered a "reasonable" use of force in these circumstances.

How about a less absurd example:

Average Joe sees an attack take place in a dark alley. As he approaches, he hears a gunshot and sees a person fall. Perhaps unwisely, he shouts and charges toward the person holding the gun. That person throws down the gun, turns, and flees. Joe runs to the wounded person, picks up the gun for safekeeping, and hears sirens. Apparently the shot was heard and someone called the police. Joe gives his accounting to the police, but the real criminal isn't caught. The victim never got a clear look at the attacker, it was dark, and it all happened so fast. Moreover, (due perhaps to the trauma of the assault) the victim's memory is fuzzy. So what do the police really have? They have Joe standing over the victim, holding the weapon. His fingerprints are on the weapon but the criminal's may not be, perhaps the attacker was wearing gloves. True, Joe doesn't have gunshot residue on his hands (or maybe he does, who knows) but perhaps the police never bothered to check before Joe washed his hands, now that evidence is meaningless. The victim doesn't know if Joe was the attacker. Perhaps Joe's story will be believed, and perhaps not. Does anyone really want to take that chance?

I'd take #3's argument even further and suggest fear of personal harm or injury. Let's be honest. There were times in American history when the outlaws and criminals (the "bad guys") were more powerful than the "good guys" (aw enforcement). The wild west days, the mob- and gang-controlled era in cities like New York and Chicago, and even--to some degree--the modern street gangs found generally on the coasts. Today, that's not as true, and law enforcement is generally able to handle the issues of crime. When it's not, communities feel the need to police themselves. If it has come to that, there is undoubtedly some danger and risk involved. While they may want to help, I'm confident many citizens are unwilling to participate out of fear.

I think that one reason a citizen might not want to be involved with community policing efforts is that there is all of the risk with none of the authority. Citizens don't carry weapons and can't make arrests. They can look out for one another and report back to the police, but they have no actual power. I imagine it would seem a bit like sifting through the ocean with a thimble, and after awhile it could get quite discouraging.